1. Field of the Invention
This invention relates to a fuel evaporative emission control system for an internal combustion engine, in particular for an engine which uses a hybrid fuel containing alcohol.
2. Description of the Prior Art
Conventional systems falling within the field of this invention include that taught by Japanese Laid-Open Patent Publication No. 58(1983)-119956, for example, which includes a canister for trapping fuel vapors to purge them into an air intake passage of the engine. In the earlier system, the alcohol concentration in the fuel is detected by an alcohol concentration sensor and the amount of purge air to the air intake passage is increased when the alcohol concentration is higher than a prescribed value and the fuel temperature is higher than a prescribed value, whereby the ratio of the fuel purged into the intake system to air is prevented from becoming too rich.
In the conventional system, when the alcohol concentration of the fuel in the fuel tank changes owing to a gasoline-alcohol blend or neat gasoline being freshly supplied to the fuel tank, the resulting difference in alcohol concentration between the fuel tank and the canister causes the air-fuel ratio of the air-fuel mixture to deviate greatly from the stoichiometric value, degrading the properties of the exhaust gas and, in the worst case, causing misfiring.
More specifically, in the case of a hybrid fuel consisting of a blend of alcohol and gasoline, when the fuel changes from a gasoline-alcohol blend such as M85 to neat gasoline, gasoline is supplied to the engine by the fuel injection valves while fuel vapors including alcohol are temporarily purged from the canister to the air intake passage. As the stoichiometric air-fuel ratio of gasoline is 14.7:1 while that of alcohol, e.g. methanol is 6.4:1, the stoichiometric air-fuel ratio of the engine changes in proportion to the amount of alcohol purged from the canister. It is also known that, inside the canister, gasoline vapors from the fuel tank are supplied into the alcohol vapors filling the interior of the canister, causing the pressure in the canister to increase owing to an azeotropic phenomenon between the alcohol and the gasoline, whereby the purge amount is more increased.